Get ready for next-gen 6Gbps SAS

Posted on November 01, 2008

The next generation of the Serial Attached SCSI (SAS) interface promises a wealth of new features, most notably a doubling of the SAS data-transfer rate from 3Gbps to 6Gbps.

A number of manufacturers have been delivering SAS components for more than a year, and the first official interoperability “plugfest” took place earlier this month (the week of November 10, at the University of New Hampshire’s InterOperability Lab). Typically, it takes 12 to 18 months after the first plugfest until products become available to end users (see figure).

However, Cameron Brett, secretary for the SCSI Trade Association (STA) and manager of product marketing at component manufacturer PMC-Sierra, predicts that 6Gbps SAS devices such as servers and external subsystems will ship to end users by the middle of next year, and notes that the exact timing may depend in part on when Intel delivers its next-generation chip sets.

Objectives

According to STA officials, the key objectives behind 6Gbps SAS were to

Double performance;

Maintain compatibility with 1.5Gbps and 3Gbps SAS and SATA;

Improve reliability, availability, and serviceability;

Reduce the number of cabling options;

Improve cost/performance and power/bandwidth ratios;

Reduce the number of connections per Gbps;

Improve link utilization; and

Improve rack-to-rack distances.

In addition, 6Gbps SAS will eventually be compatible with 6Gbps SATA.

Key features

Although there are loads of capabilities in the SAS-2 specification, the key features of 6Gbps SAS for end users and systems/storage integrators, according to Brett, are standardized expander zoning, expander self-discovery, spread spectrum clocking (SSC), mini-SAS cables, and multiplexing, which is optional (see box).

In addition to doubling performance, 6Gbps SAS extends the maximum cable length from 6 meters to 10 meters. The extended cable length will make it easier for integrators to configure “pseudo SANs,” or “SAN-in-a-rack” configurations, although proponents are not positioning 6Gbps SAS as a true SAN interconnect a la Fibre Channel.

The new spec also defines a standard method for expander zoning, which was implemented in 3Gbps SAS but in different ways by different vendors. Zoning allows multiple, non-shared devices to access and share the same domain and blocks initiators from accessing each other and any dedicated devices. The 6Gbps SAS zoning spec also provides improved security via “denial of service” and “device access control by zone” technologies. And the zoning spec also enables larger configurations, with domain segmentation.

Zoning can be done at the port, array, or device level, and the new spec supports up to 256 zones. Zoning can be particularly advantageous in virtual server environments. Harry Mason, president of the SCSI Trade Association and director of industry marketing at LSI, explains: “While SAS has the ability to address many thousands of server and storage devices, the practicality of working with even several hundred devices requires that storage devices or subsystems be consistently assigned, or zoned, to operate with multiple hosts in virtual server environments. This can be accomplished with expander zoning.”

Another expander feature in 6Gbps SAS is self-discovery; in other words, discovery can be managed by the expander itself and not necessarily by the host/controller, which reduces the amount of SAS messaging traffic and the time required to initialize systems.

Spread spectrum clocking (SSC), which varies the clock rate to spread emissions over a wider range of frequencies, is of benefit more to OEMs and integrators than to end users. SSC reduces electromagnetic interference (EMI) and makes it easier for OEMs to meet FCC requirements. SSC is required for 6Gbps implementations, but is not required for 3Gbps products.

6Gbps SAS requires the use of mini-SAS connectors (SFF-8088 and SFF-8087) for external cabling. The benefit is that this reduces cabling options (some 3Gbps SAS implementations used InfiniBand-style cabling) and improves reliability.

Multiplexing, an optional 6Gbps SAS feature, is designed to bridge the gap between 6Gbps SAS systems and 3Gbps SAS disk drives by allowing two 3Gbps SAS drives to connect to one 6Gbps SAS port. However, the value of this feature will be diminished if 6Gbps SAS disk drives are available at the same time as 6Gbps SAS systems.

The differences between, and comparative features of, 3Gbps SAS and 6Gbps SAS are summarized in the table. For users and integrators concerned about security, it is important to note that 6Gbps SAS supports features such as data integrity field (DIF) technology for end-to-end (application-host-controller-drive) data integrity/protection, and disk- and controller-based encryption.

The other table compares power/bandwidth specs of 3Gbps SAS and 6Gbps SAS, although it is important to note that the power stats for 6Gbps SAS are only estimates.

6Gbps SAS FAQs

What is 6Gbps SAS?

6Gbps SAS is the marketing name that the SCSI Trade Association (STA) has given to the industry standard based on the SAS–2 specification. It is the next generation of the SAS protocol beyond the current 3Gbps SAS. One of the key benefits of SAS is its compatibility with the SATA interface, which enables tiered storage solutions. The first generation of SAS took advantage of features such as dual-ported drives, wide ports, and full-duplex transmission to provide a performance advantage over parallel SCSI. 6Gbps SAS doubles the bandwidth.

What is the distinction between 6Gbps SAS and the SAS-2 specification?

SAS-2 is the name of the latest T10 SAS specification. In SAS-2, 6Gbps signaling is a component of the overall specification. It specifies all of the new features being incorporated into the technology, as well as how to implement these features.

The 6Gbps SAS transfer rate doubles system throughput over 3Gbps SAS. For example, using SAS as the connection between a host and an external storage controller, a wide (x4) full-duplex SAS channel enables data transfers of 24Gbps in both directions.

What new benefits will 6Gbps SAS bring to the enterprise storage market?

Multiplexing is an optional feature of 6Gbps SAS and is being selectively implemented in 6Gbps products. It offers an optimized 6Gbps SAS infrastructure for a subset of applications requiring 3Gbps SAS components. The advent of 6Gbps SAS drives will limit the utility of this feature. If implemented, multiplexing must conform to the SAS-2 specification definition.

6Gbps SAS standardizes how zoning is done. The zoning standard for multiple hosts provides the capability to divide a storage pool across multiple hosts. This is important in order to virtualize computing and blade system applications. Zoning provides the ability to restrict large topologies by limiting the storage that is visible to the system at power-up, improving the boot time.

Yes, 6Gbps SAS maintains compatibility with existing SCSI software and middleware. In addition, it is compatible with the hardware in existing SAS installations. Electrical protocols, cables, and mini-SAS connectors (4x and 4i) are all compatible with previous generations.

What other connectivity features are in 6Gbps SAS?

Use of decision feedback equalization (DFE) with 6Gbps SAS signaling permits longer cables than 3Gbps SAS. SAS also has potential as a fabric with the use of high-port-count expanders to create large, yet simple and cost-effective, tiered storage environments.

What are the benefits of self-configuring expanders?

Discovery is executed by SAS expanders, which reduces the time required to discover large topologies, thereby enabling larger and more complex topologies.

What is the 6Gbps SAS delivery timeline?

6Gbps SAS prototypes were introduced in Q3 2007. The first official 6Gbps SAS plugfest took place the week of November 10, 2008. Initial components, such as HBAs and backplanes, started to roll out in the first half of 2008, with production components delivered in the second half of the year. Storage systems, such as servers and external storage subsystems, will begin shipping in mid-2009.

Source: SCSI Trade Association

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